1. Field of the Invention
The present invention is directed to a method for producing a directional microphone characteristic in a hearing aid device of the type having at least two input signal paths each with an omnidirectional input transducer and a high-pass filter following the input transducer, a signal pre-amplifying unit, a signal processing unit, a signal output amplifier and an output transducer, with at least two of the input signal paths are interconnected with one another to produce the directional microphone characteristic.
2. Description of the Prior Art
A hearing aid device having two omnidirectional input transducers, referred to in the following also as microphones, is known from European Application 848 573. A series-connected microphone, coupling capacitor and resistor are, respectively, located in two separate signal paths which are interconnected with one another to produce a directional microphone characteristic. In addition, one of the two signal paths has a signal delay unit. A disadvantage of this known circuit is that the desired directional characteristic can be attained only if the two microphones deviate at the most only negligibly from one another with regard to their signal transmission behavior. In the output signal of the two microphones, a phase difference of more that 3xc2x0 in the frequency range in which the directivity is to be attained already acts negatively on the desired directional characteristic of the arrangement. Only microphones having almost the same signal transmission behavior thus can be used in the known circuit. Since, however, larger manufacturing tolerances cannot be avoided in the manufacture of the microphones, two microphones matching one another, i.e. exhibiting the same signal transmission behavior, must be selected from a larger number of similar microphones. This process is time-consuming and costly.
A circuit is known from the article published in March 1999 xe2x80x9cElectrical Compensation of the Microphone Sensitivities in a Dual Microphone Directional Hearing Aidxe2x80x9d by Stephen C. Thompson, Knowles Electronics Inc., that enables a correction of the phase difference of the output signals of two microphones inserted into the signal paths of the two microphones. This circuit is, however, complicated and requires additional electrical components in the two signal paths.
An object of the present invention is to provide a method for producing a directional microphone characteristic in a hearing aid device having two microphones of the same type, that deviate from one another in their signal transmission behavior. In addition, an object of the invention to provide a hearing aid device wherein it is possible, in a simple and economical manner, to achieve a directional microphone characteristic with two microphones of the same type that deviate in their signal transmission behavior.
The above object is achieved in accordance with the principles of the present invention in a method for producing a hearing aid with a directional microphone characteristic, and a hearing aid produced according to the method, wherein at least two omnidirectional microphones are used to receive incoming acoustic signals and wherein each microphone has a signal path connected therewith for processing the signals received by that microphone, the signal paths subsequently being combined to form an overall output signal which is supplied to an output transducer, and wherein each signal path has a high-pass filter therein, with the respective limit frequencies for the high-pass filters being set to match the respective limit frequencies of the microphone in the other signal path. In a hearing aid having two such signal paths, for example, the limit frequency of the high-pass filter in a first of the signal paths is matched to the limit frequency of the omnidirectional microphone which is connected in the second of the two signal paths, and the limit frequency of the high-pass filter in the second of the frequency paths is matched to the limit frequency of the omnidirectional microphone in the first of the signal paths.
The inventive method allows two identical microphones that deviate from one another with respect to their amplitude- and/or phase-response to be matched to one another in a simple and cost-expedient manner such that the desired directional microphone characteristic is attained. For this purpose the values of the coupling capacitors and/or resistors that respectively follow the microphones in the two signal paths of the microphones are adapted to the microphones. In contrast to known devices, there are no additional components to be adapted according to the invention; rather, it suffices either to use components having fixed values adapted to the microphones or to provide components having modifiable values for the capacitors and/or resistors and to subsequently match these, e.g. via programming, to the microphones used. A coupling capacitor and a series-connected resistor are customary coupling-in a microphone output signal and, consequently, are not additional components. The signal behavior of a coupling capacitor and a resistor in the described manner conforms to a high-pass filter.
Microphones customarily used in hearing aid devices nowadays represent acoustic high-pass filters in their signal transmission behavior. The limit frequency of such a high-pass filter, i.e. the frequency at which the magnitude of the output signal divided by the magnitude of the input signal equals xe2x88x923 dB, is about 100 Hz. To reach this limit frequency, each of the microphones used has a small hole in its membrane, causing the limit frequencyxe2x80x94dependent on the diameter of this hole in the membranexe2x80x94to be shifted to higher values. This shift is necessary to suppress interference signals of lower frequency, as occur in a car, for example, which otherwise could easily lead to over-amplification in the hearing aid device.
Consequently, an acoustic signal undergoes a filtering in two successive high-pass filters and is correspondingly changed as a result in its amplitude response and phase response.
In a directional microphone arrangement, it is necessary for the signal paths of the individual microphonesxe2x80x94in particular for low frequenciesxe2x80x94to not only match the amplitude responses, but also, above all, the phase responses that are determined very strongly by the limit frequency of the successive high-pass filters.
The amplitude and/or phase balancing of two omnidirectional microphones of the same type, the signals of which are appropriately interconnected for producing a directional microphone characteristic, is attained in the invention by balancing their limit frequencies. This occurs in a particularly simple manner for two microphones that together form a directional microphone, by matching the limit frequencies of the high-pass filters following the microphones, formed by at least one coupling capacitor and a resistor, to the limit frequencies of the microphones, in such a manner that limit frequency of the microphone of one signal path corresponds to the limit frequency of the high-pass filter (following the microphone) of the other signal path.
In an embodiment of the invention, the setting of the limit frequencies of the high-pass filters following the microphones ensues by adjusting the capacitor and/or resistors having variable values. This has the advantage that the capacitors and/or resistors do not have to be specified before the hearing aid device is assembled. In addition, a subsequent readjustment is possible.
The values of the variable resistors and/or capacitors roughly correspond in orders of magnitude to the values of the non-variable resistors and/or capacitors in hearing aid devices according to the prior art. Consequently, they can be realized and integrated into the circuit without difficulty.
In another embodiment of the invention, the limit frequencies of the high-pass filters following the microphones are set by correspondingly programmable resistors and/or capacitors. Thus the microphones can be balanced in a simple manner by programming the hearing aid device. The setting of the limit frequencies can ensue based on the data of the microphone manufacturer regarding the limit frequency of the respective microphone; it can also be implemented at an adjustment station suitable therefor.
Since, e.g. a change in the resistance values influences the subsequent signal amplification, an adaptation of the amplification is necessary to restore the desired weighting of the two signal paths as they merge. In addition, the signal path having the delay element includes an attenuation element following the delay element, preferentially in the form of a variable value resistor.
The invention is employable for all hearing aid embodiments and technologies, e.g. for behind-the-ear or in-the-ear hearing aid devices or implantable hearing aids that can be constructed in analog or digital circuit technology or in hybrid forms.